This invention relates to screen process printers of microcircuits, and more particularly, to a squeegee comprising an essential part of the screen process printing apparatus.
A form of screen printing which is widely used today, particularly in the production of microelectronic circuitry is known as "thick film" printing. In general, most of the screen printers in use are flat bed devices with a horizontal work carrier in squeegee travel attitudes. Usually, the screen is fixed, while the squeegee assembly moves in respect thereto. The circuit or portion of a circuit or layer of dielectric is printed on a substrate which is usually in the form of a flat wafer. The components of the circuit are printed on the wafer by using a slurry, which is often called ink or paste, the paste being a dielectric paste or a metallic paste. A microelectronic pattern is created by forcing the paste through a screen containing the same pattern of open mesh in the screen and which covers the substrate material. This is often accomplished by first applying the paste over the screen and then moving a blade, usually known as a "squeegee", over the screen so that the squeegee pushes the paste forward and hydraulically forces the paste into the pattern opening of the screen so that the paste passes onto the substrate. Thus, the paste in the pattern opening has been sheared from the paste on the top of the screen by the blade, and the screen upon retracting leaves the image paste on the top of the substrate because the paste has greater affinity for the substrate than for the screen. The result is the deposition of an image, i.e., a layer of printed material approximately the thickness of the screen. If the miniaturized circuit is to function properly, parameters of the circuit must be within acceptable limits. This means that the size of the thickness of the printed component must be within predictably narrow limits and since the circuitry is highly miniaturized, the tolerances are miniscule.
The squeegee pressure on the screen and substrate and the uniformity of the pressure is vital to the formation of acceptable limits. Too little pressure will prevent proper seals between the squeegee and the screen and between the screen and the substrate which are necessary for precision printing. Present systems include various ways for maintaining the force between the squeegee and the printing screens, for example, biasing the squeegee towards the screen by means of a coil spring, or utilizing a torsion bar and link to exert a moment on the link and thus force the squeegee to urge the squeegee against the printing screen, as described in U.S. Pat. No. 3,924,529, entitled "Torsion Bar Floating Squeegee Mount for Screen Printer".
In existing systems, the squeegee has taken a variety of forms. For example, a flat squeegee stock has been employed, and is disposed in a position of inclination with respect to the plane of the screen, so as to apply only a corner of the stock to the screen. Or, in another flat form, the edge is beveled with the squeegee being disposed in a plane perpendicular to the plane of the screen. In every instance, the squeegee is intended to present a comparatively even, sharp edge to the screen, and to define an angle of 45.degree. to 60.degree. between the leading surface of the squeegee and the screen, thus to push a roll of paste ahead of the squeegee while at the same time creating a pressure upon the paste forcing it through the screen (reference U.S. Pat. No. 3,438,819).
The present invention provides for a novel cross-sectional configuration of the squeegee which increases the pressure of the blade against the screen, especially desired in today's "fine-line" technology where line widths are in the 1-mil range, thereby creating more reliable printing of the paste onto the substrate.